The Drosophila S2 cell-line is potentially a very useful system for production of therapeutic proteins for human use. Its attributes include both high-eukaryotic post-translational modification and protein folding pathways as well as high level expression as a free-floating cell suspension in bioreactors. These cells contain virus-like particles that present a substantial hurdle for FDA approval of any human therapeutics derived from this system. These particles are derived from multiple Copia retrotransposons in the Drosophila genome, we propose to eliminate these particles using expression of a repressor protein designed to block Copia transcription. Phase I research will start with construction of zinc finger protein DNA-binding modules directed against two 18-bp sites within the Copia element. Design of the repressor protein will be facilitated by direct screening of small libraries tailored to recognize the target sequences through rational design. The design process is direct, based on extensive analysis of DNA recognition by zinc finger proteins. The resulting DNA- binding modules will be tested in S2 cells with and without a fused trans- acting repressor domain. We anticipate repressor protein binding to one target site will directly block transcription factor binding, while repression from the other site is expected to require the trans-acting domain. PROPOSED COMMERCIAL APPLICATIONS: The Drosphila Schneider S-2 cell line is one of the most productive eukaryotic protein expression systems. Cells cured of virus-like particles would be advantageous for production of proteins for human therapeutic use. Such cells could be used by HBG to produce proteins it currently has in development and could be licensed to other companies for production of their products.